There are, at least, two splicing variants of CaMKII-d, dB and dC, located in unclear and cytosol compartments, respectively. Our present in vivo and in vitro studies have provided multiple lines of evidence to demonstrate that CaMKII-dB and CaMKII-dC exhibit opposing functional roles in regulating cardiomyocyte viability with CaMKII-dB protective and CaMKII-dC apoptotic. [unreadable] (1) CaMKII Activation Is Required for b1AR-Induced Apoptosis in Cardiomyocytes and In Vivo. [unreadable] Sustained b1AR stimulation markedly increased CaMKII activity in a time-dependent manner in adult mouse cardiomyocytes; this effect was abolished by a specific CaMKII inhibitors AIP or KN93 but not by specific PKA inhibitors Zhu et al.,J. Clin. Invest. 111:617-625, (2003). Importantly, inhibition of CaMKII fully protected myocytes from b1AR-induced apoptosis. In collaboration with Mark Anderson, we further demonstrated that CaMKII was essential for in vivo apoptotic response to excessive catecholamine stimulation Yang et al., Am. J. Physiol. 291:H3065-H3075, (2006).[unreadable] (2) Increased CaMKII-dC Activity Is Sufficient to Cause Heart Muscle Cell Apoptosis.[unreadable] We specifically enhanced or inhibited CaMKII-dC activity by adenoviral gene transfer of a constitutively active (CA- CaMKII-dC) or a dominant negative CaMKII-dC mutant (DN-CaMKIIdC), respectively. Enforced expression of CA-CaMKII-dC alone caused increased cardiac myocyte apoptosis. The severity of cell apoptosis was closely correlated with CA-CaMKII-dC protein abundance and the kinase activity, suggesting there is a causal relation between activation of CaMKII-dC and cardiomyocyte apoptosis.[unreadable] (3) Activation of Endogenous CaMKII by Various Stimuli That Trigger Myocyte Apoptosis[unreadable] Multiple cell death-inducing stimuli, such as increased intracellular Ca2+ concentration, acidosis, and oxidative stress, increased endogenous CaMKII activity by 23-fold over baseline. These stimuli markedly triggered myocyte apoptosis in CaMKII inhibitors sensitive manner. [unreadable] To define the relative contributions of CaMKII-dC versus that of CaMKII-dB to the observed cell death, we suppressed CaMKII-dC activity using the DN-CaMKII-dC, and found that expression of DN-CaMKII-dC inhibited the kinase activity and the associated cell death. These results indicate that activation of CaMKII-dC constitutes a common pathway converging multiple stimuli evoked apoptotic signals in cardiomyocytes Zhu et al., J. Biol. Chem. 282:10833-10839, (2007).[unreadable] [unreadable] (4) CaMKII-dB Protects Cardiomyocytes Against Apoptosis.[unreadable] CaMKII-dB expression is remarkably attenuated at both mRNA and protein levels in rat ischemia/reperfusion (I/R) and myocardium infarction (MI) models and in cultured cardiomyocytes subjected to oxidative stress with H2O2. The inhibitory effects of MI and H2O2 on CaMKII-dB are fully prevented by ROS scavengers, indicating ROS constitutes a negative regulator of CaMKII-dB gene expression. Concurrently, MI and H2O2 markedly increase myocyte apoptosis in vivo and in culture, respectively, assayed by Hoechst or TUNEL staining and caspase activation. Most importantly, overexpression of CaMKII-dB using adenoviral gene transfer protects heart cells against oxidative stress-induced apoptosis, assayed by DNA laddering and PI staining. The CaMKII-dB protective effect is mediated by an HSP70-mediated signaling pathway. [unreadable] [unreadable] Based on the opposing effects of these cardiac CaMKII isoforms, we envision that a combination of activation of CaMKII-dB with inhibition of CaMKII-dC should be superior to isoform-nonselective inhibition of CaMKII as a potential therapy for the treatment of heart failure or cardiac arrhythmia.